This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Quorum sensing is a bacterial cell-cell communication process that allows groups of bacteria to act in unison, synchronizing important phenotypic changes. One family of secreted quorum-sensing signals, called Phr pentapeptides, is imported into the cell where they inhibit the activity of proteins called Raps. To determine how Rap proteins function mechanistically, and how Rap functions are in turn regulated by Phr peptides, we are studying two subsets of Bacillus subtilis Rap proteins. One Rap protein subset negatively regulates sporulation in B. subtilis by dephosphorylating a central protein in the sporulation signal transduction pathway, the other subset of Rap proteins downregulates the development of genetic competence by preventing the master transcriptional regulator of early competence gene expression from binding to target DNA promoters. We have crystallized Rap proteins in complex with their target effector proteins, as well as with their regulatory Phr peptides. The X-ray crystal structures of these complexes, combined with biochemical and genetic studies, will reveal for the first time how Rap proteins function mechanistically and how Phr peptides regulate Rap protein activity.